@davidjames -- I can't tell you which is "best" but we do have free AV for Android if you'd like to give it a try.
The truth is that we've seen numerous Android threats, and they are growing. Everything from fake apps that download malware payloads, to banking trojans, and more recently cryptocurrency miners that operate without your permission (cryptojackers.) Some of these are quite nefarious and even imitate legitimate prompts to try to trick you into entering your credentials or giving admin access to your device.
This is a really good example of how deep these threats can go, and in this case the apps actually passed through Google Play before they were found and removed. They were "droppers," fake apps that were not malicious...